import sys
from collections import defaultdict

def main():
    n = int(sys.stdin.readline())
    a = list(map(int, sys.stdin.readline().split()))
    count_map = defaultdict(int)
    for num in a:
        count_map[num] += 1

    ans = 0
    for num in a:
        target = num ^ 1
        ans += count_map.get(target, 0)

    print(ans // 2)

if __name__ == "__main__":
    main()